This sort of thing seems to happen with iPhone apps in general - not exactly sure what causes it, but general first step to deal with it is to de-install the application (hold down the home key until the icons "jiggle" and tap the "X" button on the icon) and then re-sync with iTune to have it reinstall the application.

Basically something somewhere is corrupted during the install and so the built in authentication/crypto code signature stuff prevent it from running.